1. Field of the Invention
This invention relates generally to the collection and control of light from a light emitting diode (LED). More specifically, the invention is directed to the control of the wide angular emission of light from an LED to create a highly controlled beam of light.
2. Background Art
Numerous products require efficient collection and control of light from a light source. A high degree of control is required to create a collimated beam of the type needed for searchlights used for live performances, special events, and illuminating tall structures. The searchlights use a xenon arc type lamp as a light source and a deep parabolic reflector to collect and control the light direction and beam angle. This method has been used for many years, even before the invention of the light bulb.
This type of prior art searchlight generally requires a reflector that is much larger than its arc gap. A 1000-watt xenon lamp generates most of its light within a sphere 1 mm in diameter. To create a highly collimated beam, a reflector with a diameter of 20 inches is typically used. Although xenon light sources create an enormous amount of light in a small area, efficiency of these types of lamps is poor. A 1000-watt lamp may only produce 35 lumens per watt of electrical energy. Another drawback with these lamps is the length of their life, which is only a few thousand hours. Finally, xenon light sources are filled with gas at a high pressure. Persons replacing xenon lamps need to wear protective clothing and a face shield when they are servicing searchlights.
Another disadvantage of the xenon light systems is the reduction in performance as a result of the collection of dirt on the optical surfaces. This collection is compounded by the fact that the lights typically require forced-air cooling. A xenon system has at least four surfaces where dirt can collect and reduce the output. The first of these surfaces is the surface of the lamp itself. The second is the surface of the reflector. The third and fourth are the inside and outside of the window. Only a small amount of dirt on any of these four surfaces significantly reduces the light output of the system
The use of a deep parabolic reflector by searchlight manufacturers adds to the poor efficiency of the overall system. A lot of the light generated from the lamp exits the front of the open end of the parabolic reflector and doesn't contribute to the collimated beam created by the light that does strike the reflector.
Manufacturers of searchlights would like to use LEDs as the light sources for their searchlights. LEDs don't create light with as high intensity as the xenon light sources. The low intensity of LED light leads to the requirement of a much larger reflector for the same output as a xenon system. In some cases using an LED would require a reflector ten times the size of the reflector used by a system with a xenon light source. In summary, the main disadvantages of current xenon based light systems are their short life, the dangers of servicing the systems, and their low efficiency.
Optics systems to collect and control light from LEDs commonly combine a conventional reflector and refractive optics. A typical example of this type of system is shown in FIG. 1. Although this type of system is efficient in collecting all of the light from the LED, the ability to control the output is limited. The light that is collected by the reflector portion of the system has a generally uniform cone angle as it leaves the reflector. In this example the cone angle ranges from 3.9 degrees to 4.5 degrees. The refractive optics (i.e. the light transmitted through the lens) has a much greater cone angle, 41 degrees. Therefore, in searchlight systems, the light from the refractive optics does not contribute to the searchlight beam and creates spill light.
Another drawback inherent to the prior art system of FIG. 1 is that the output light comes from two sources, a lens and a reflector. The nature of the light from the lens is quite different from that from the reflector. It is therefore very difficult to optimize the output from both sources simultaneously. Output controlling measures that have a positive effect on the light output from the lens tend to have a negative effect on the light output from the reflector, and vice versa.
Another variation of conventional reflector optics is a lamp that locates an LED at the focal point of a parabolic reflector. The output normal to the surface of the LED is directed along the axis of the parabola. Light from the LED is emitted in a semispherical direction, + and −90 degrees from normal. The parabola collects all of the emitted light and directs most of the light in the intended direction. The LED and its mounting absorb some of the light that would, if not obstructed, go in the intended direction. This absorption occurs because the LED is in the output path of the light reflected by the parabola.
Electricity must be supplied to the LED to generate the light, which creates heat. To cool the LED, a heat pipe is used to conduct heat from the LED to a heat sink behind the reflector. These components also absorb some of the light, thereby reducing the efficiency of the lighting system even further.
The reflector in an LED light system needs to be large to collect the semispherical, ±90 degree output from the LED. If the cone angle could be reduced to less than ±45 the reflector could be much smaller. The output beam angle of the reflector of prior art products varies greatly as a function of the distance from the center of the reflector to the rim of the reflector. The variation in beam angle requires the reflector to be larger than would be required if the variation in beam angle over the diameter of the reflector was reduced.
There is therefore a need for a lighting system that is highly efficient, that is not as sensitive to dirt and dust, that provides a high degree of control of the output beam angle, and that is contained in a compact package.